know sth backwards and forwards 这个backward是形容词，加了s是什么用法呢？

这里backwards and forwards是词组
backwards and forwards
[ˈbækwədz ænd ˈfɔ:wədz]
adv.
来回地
例句：
1、 Draw the floss backwards and forwards between the teeth.
将牙线在齿间来回拉动。
2、 He turned on the light and began to pace backwards and forwards.
他打开电灯并开始走来走去.
3、 The shuttle moves backwards and forwards through the warp.
梭子在经线中来回移动.
4、 Travelling backwards and forwards everyday cost him too much money.
每天来回花了他太多的钱.
5、 He lit the gas and began to pace backwards and forwards.
他点燃了煤气炉,然后开始来回地在地上踱着步.